1. Field of the Invention
The present invention relates to mining and construction vehicles, and more particularly, to suspension systems for such vehicles.
2. Description of the Prior Act
Heavy duty mining and construction vehicles typically have a suspension system coupling the vehicle frame to the axles, which allows the axles to float or oscillate relative to the frame. In some designs, the suspension system is operative only when the vehicle is being driven to or from a construction or mining site. Once the work site is reached, the suspension system is locked to secure the axles relative to the frame to increase the stability of the vehicle.
A particular type of vehicle for which locking suspensions are useful are cranes which have a high center of gravity. A crane's center of gravity also shifts with the movement of the boom. Accordingly, if the frame of the crane is allowed to move relative to the vehicle axles while the boom is moving, the crane may become unstable and, as a result, the crane may tip over. To avoid this problem, U.S. Pat. No. 4,264,014 suggests a device which unlocks the suspension system only when the boom of the crane is pointed straight forward i.e., the position of the crane when the vehicle is being driven. In that position, the vehicle may be driven with the suspension system operative.
Thus, there have been systems suggested for activating the suspension system when the vehicle is ready to be driven and for locking the suspension system once the crane is put into use at the next site. However, these previous suspension control systems typically do not actively control the suspension while the vehicle is being driven. This is particularly disadvantageous for articulated vehicles which have a front frame pivotally coupled to a separate rear frame. When an articulated vehicle turns, the front frame and rear frame pivot relative to each other, which can sharply reduce the turning radius of vehicle and make it considerably more maneuverable. However, the pivoting of the front and rear frames also shifts the center of gravity of the vehicle. As is more fully explained below, if one or more of the vehicle axles is allowed to oscillate while the vehicle is moving in order to maintain contact between the wheels and the ground over rough terrain, the range of the allowable shift in the center of gravity is reduced. If the center of gravity shifts beyond this allowable range, the vehicle may tip over.